Conventionally, a wireless communication system using a plurality of wireless stations that includes wireless communication units transmitting and receiving wireless signals, respectively, has been put to practical use. For example, each of Patent Documents 1 and 2 discloses an audio and video data wireless delivery system (the audio and video data will be referred to as an AV data hereinafter) in an aircraft. Patent Document 3 proposes a wireless communication system using a wireless local area network (the local area network will be referred to as a LAN hereinafter) in a vehicle. As a wireless communication unit of the wireless LAN system, the products compliant with the IEEE802.11 standard (See, for example, Non-Patent Document 1), for example, has been put to practical use and marketed. The IEEE802.11 standards contain a plurality of types of standards such as IEEE802.11a, IEEE802.11b, IEEE802.11g, and IEEE802.11n, that are decided according to utilization frequency, transmission method or the like.
As an access control method used in these wireless LAN systems, CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) has been adopted. With this method, one wireless station detects the radio wave from the other wireless station before transmitting packets from its own wireless station. If the wireless station detects that the other wireless station emits the radio wave, the wireless station withholds from transmitting the radio wave for a certain period of time and checks again whether or not the other wireless station emits the radio wave. If the wireless station detects that the other wireless station does not emit any radio wave, the wireless station transmits the radio wave. Namely, with this method, one wireless station is influenced by the radio wave from the other wireless station. In particular, when a plurality of wireless stations use the same channel, the frequency bands used become completely overlapped on each other. Therefore, such a problem occurs that one wireless station is remarkably influenced by radio waves from the other wireless stations and incapable of transmitting the radio wave from its own wireless station. This eventually attributes to a decrease in effective band and a considerable degradation in communication quality. Concretely, if a plurality of “n” wireless stations in the same channel are present in an interference range, an effective band of each of the wireless stations is probabilistically equal to or narrower than 1/n.
In addition, the IEEE802.11 standards define adjacent channel rejection, which indicates an allowable level of an interference wave of an adjacent channel, namely, at how higher level than that of a channel to be received. If one channel is strongly interfered with an adjacent channel, then a waveform (frequency characteristic) of a received signal changes, and packets cannot be normally received, resulting in packet error. The results attribute to the decrease in effective band and the considerable degradation in communication quality. Non-Patent Document 2 describes the CSMA/CA and the adjacent channel rejection in detail.
FIG. 15 shows frequency bands, central frequencies, and wireless channel numbers of 5 GHz bands (F1 and F2) used according to the IEEE802.11a standard. FIG. 16 shows frequency bands, central frequencies, and wireless channel numbers of 5 GHz bands (F3 and F4) used according to the IEEE802.11a standard. In respective embodiments of the present invention to be described later, a method of allocating wireless channels using frequency bands specified under the IEEE802.11a standard will be described.
Currently, the wireless channels available under the IEEE802.11a standard include the following:
(a) a group of eight continuous channels from the channel 36 to the channel 64;
(b) a group of eleven continuous channels from the channel 100 to the channel 140; and
(c) a group of four continuous channels from the channel 149 to the channel 161. Some countries are considering using the wireless channels other than these channels.
It is only in the United States that permitted to use the group (F4) of four continuous channels from the channel 149 to the channel 161. The channels to be currently discussed to be allowed universally all over the world include nineteen channels in all from the channel 36 to the channel 64, and from the channel 100 to the channel 140. In the embodiments of the present invention, an instance of using the channel 36 to the channel 64 will be described by way of example.
In a wireless communication system using the CSMA/CA, it is mainly the same channel interference that influences communications among wireless stations and adjacent channel interference also influences the communications among wireless stations. The same channel interference occurs when, for example, one wireless station uses the channel 36 and the other wireless stations in an interference range uses the same channel 36 in FIGS. 15 and 16. Likewise, the same channel interference occurs when one wireless station uses the channel 40, 44, 48, 56, 60 or 64 and the other wireless stations use the channel of the same number. In addition, the adjacent channel interference occurs in the following cases:
(a) when the channel 40 is used for the channel 36;
(b) when the channels 36 and 44 are used for the channel 40;
(c) when the channels 40 and 48 are used for the channel 44;
(d) when the channels 44 and 52 are used for the channel 48;
(e) when the channels 48 and 56 are used for the channel 52;
(f) when the channels 52 and 60 are used for the channel 56;
(g) when the channels 56 and 64 are used for the channel 60; and
(h) when the channel 60 is used for the channel 64.
Patent Document 4 discloses a method of reducing interference among wireless stations. With this method, the different frequency bands are used at least in an adjacent wireless zone interval among the respective wireless zones.
Further, Patent Document 5 discloses a method of preventing interference between a fixed station and a base station. This method is an interference reduction method if cells are present in a range of 180 degrees relative to a specific direction on the horizontal plane. With this method, the different frequencies or polarized waves are used between adjacent sectors of adjacent cells.
Moreover, Patent Document 6 discloses a method of reducing adjacent channel interference. With this method, the mobile stations having almost equal received field intensities are grouped, the wireless signals are transmitted to the grouped mobile stations in a frequency division or time division multiple access, and this leads to reduction in the adjacent channel interference.
In addition, Patent Document 7 discloses a method of reducing interference if, for example, difference systems are present in the same floor or the like. With this method, the frequency channels used by respective systems to be used are classified into a plurality of groups according to the systems, and this leads to preventing from occurrence of interference.
Patent Document 1: Japanese patent laid-open publication No. JP-2006-506899-A.
Patent Document 2: U.S. patent laid-open publication No. 2004/0098745.
Patent Document 3: Pamphlet of International Publication No. WO03/032503.
Patent Document 4: Japanese patent laid-open publication No. JP-7-212828-A.
Patent Document 5: Japanese patent laid-open publication No. JP-2002-112318-A.
Patent Document 6: Japanese patent laid-open publication No. JP-2003-209508-A.
Patent Document 7: Japanese patent laid-open publication No. JP-7-336363-A.
Non-Patent Document 1: ANSI/IEEE Std 802.11, 1999 Edition (R2003), “Information technology—Telecommunications and Information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, LAN MAN Standards Committee of the IEEE Computer Society, Reaffirmed on 12 Jun. 2003, IEEE-SA Standards Boards, IEEE (Institute of Electrical and Electronics Engineers, Inc.) Standards Association, [Searched on Jun. 29, 2006], Internet <URL: http://standards.ieee.org/getieee802/802.11.html>.
Non-Patent Document 2: Editorial Supervisors of MATSUE Hideaki et al., “802.11 High-Speed Wireless LAN Textbook”, First Edition, IDG Japan, Inc., Mar. 29, 2003.